1. Field of the Invention
The present invention relates to a spacer grid for supporting nuclear fuel rods of a nuclear fuel assembly charged into a nuclear reactor and, more particularly, to a spacer grid for more close-spaced nuclear fuel rods than conventional ones, in which the supports of each grid strap are located at positions shifted from a central line of each grid strap toward a center of a sub-channel in order to accommodate a reduced gap between the nuclear fuel rods due to the use of dual-cooled nuclear fuel rods, which have excellent cooling performance.
2. Description of the Prior Art
FIG. 1 is a perspective view schematically illustrating a conventional nuclear fuel assembly. FIG. 2 is a cross-sectional view schematically illustrating a conventional nuclear fuel assembly. FIG. 3 is a top plane view schematically illustrating a spacer grid applied to a conventional nuclear fuel assembly. FIG. 4 is a perspective view schematically illustrating a spacer grid applied to a conventional nuclear fuel assembly. FIG. 5 is a perspective view schematically illustrating a unit grid strap for a spacer grid supporting a conventional nuclear fuel assembly.
As illustrated in the figures, the conventional nuclear fuel assembly 100 comprises nuclear fuel rods 110, guide tubes 140, spacer grids 150, a top end piece 120, and a bottom end piece 130.
Here, each nuclear fuel rod 110 has a cylindrical uranium sintered compact in a clad pipe of zircaloy (zirconium alloy). This uranium sintered compact is fissioned to generate high temperature heat.
Meanwhile, each guide tube 140 is used as a passage for a control rod, which moves up and down in order to control the output power of a reactor core and to stop the fission reaction. The spacer grid 150 is one of the components constituting the nuclear fuel assembly in a nuclear reactor, and includes a plurality of unit grid straps, each of which has a spring 118 and dimples 119 and functions to support and protect the nuclear fuel rods 110 so that they are arranged at designated positions. When the spring force of the spring 118 and the dimples 119 is too weak, each nuclear fuel rod 110 cannot be arranged at a designated position, and thus has a possibility of losing sound supporting performance. In contrast, when the spring force of the spring 118 and the dimples 119 is too strong, each nuclear fuel rod 110 undergoes defects such as scratching on the surface of the clad tube due to excessive frictional gripping force when it is inserted into the spacer grid. Further, during the operation of the nuclear reactor, the nuclear fuel rods 110 experience longitudinal growth by means of the irradiation of neutrons. This longitudinal growth is not properly accommodated, and thus the nuclear fuel rods 110 are bent. In this manner, when the nuclear fuel rods 110 are bent, they come nearer to or contact neighboring nuclear fuel rods 110. Thus, the coolant channel between the nuclear fuel rods becomes narrow or is blocked. As a result, the heat generated from the nuclear fuel rods is not effectively transmitted to the coolant, thereby increasing the temperature of the nuclear fuel rods. As such, the possibility of generating Departure from Nucleate Boiling (DNB) is increased, which is mainly responsible for the reduction of nuclear fuel output power.
The top end piece 120 and the bottom end piece 130 fixedly support the nuclear fuel assembly 100 on upper and lower structures of the reactor core. The bottom end piece 130 includes a screening device (not shown) for filtering foreign materials floating in the reactor core.
Meanwhile, each spacer grid 150 is usually made of zircaloy, and includes nuclear fuel rod cells, which support the nuclear fuel rods 110, and guide tube cells, into which the guide tubes 140 are inserted. Each nuclear fuel rod cell is designed to support each nuclear fuel rod 110 at a total of six supporting points using a total of two grid springs 118, which are located on two respective faces, and a total of four dimples 119, which are located in pairs above and below the two grid springs 118 and on the other two respective faces.
A cylindrical uranium dioxide compact is inserted into each nuclear fuel rod 110, and the coolant rapidly flows from the bottom to the top of the reactor core in an axial direction through sub-channels 115, each of which is enclosed by four nuclear fuel rods 110 or by three nuclear fuel rods 110 and one guide tube 140.
Here, each sub-channel 115 refers to a space enclosed by the nuclear fuel rods 110, and particularly a passage through which a fluid can freely flow to the neighboring sub-channel because it has an open side.
Meanwhile, as illustrated in FIGS. 6 and 7, a dual-cooled nuclear fuel rod 10 having an annular structure instead of the cylindrical nuclear fuel rod 110 is disclosed in U.S. Pat. Nos. 3,928,132 and 6,909,765.
Here, the dual-cooled nuclear fuel rod 10 having an annular structure includes a sintered compact 11 having an annular shape, an inner clad tube 12 enclosing the inner circumference of the sintered compact 11, and an outer clad tube 13 enclosing the outer circumference of the sintered compact 11. Thus, the coolant flows outside and inside the dual-cooled nuclear fuel rod 10, so that heat transfer is doubled. As a result, the dual-cooled nuclear fuel rod 10 can maintain a low fuel's centerline temperature, and provide high combustion and high output power.
In this manner, in the case in which the centerline temperature of the dual-cooled nuclear fuel rod 10 is maintained low, the possibility of damaging the fuel due to an increase in the core temperature of the nuclear fuel is lowered, so that the safety allowance of the dual-cooled nuclear fuel rod 10 can be increased.
However, in order to make the dual-cooled nuclear fuel rods 10 structurally compatible with an existing pressurized water reactor (PWR) core, the gap between the nuclear fuel rods becomes considerably narrower compared to that between existing nuclear fuel rods because the positions of the guide tubes 140 cannot be changed in the nuclear fuel assembly 100, and because the outer diameter of each nuclear fuel rod is increased. For example, in the case in which the nuclear fuel assembly is formed according to a candidate design draft for the dual-cooled nuclear fuel rods having a 12×12 array, the gap between the nuclear fuel rods is reduced from 3.35 mm, which is the size of the existing gap, to about 1.24 mm.
Thus, due to the narrow gap between the nuclear fuel rods, the spacer grid that has been developed to date cannot be used as that for the dual-cooled nuclear fuel rods 10.
In other words, after the thickness of the unit grid strap of the existing spacer grid, which is 0.475 mm, is subtracted from the gap of 1.24 mm between the nuclear fuel rods, the obtained result is again divided by two. Consequently, the gap between the unit grid strap and the nuclear fuel rod is no more than about 0.383 mm. Thus, it is impossible to apply such a shape and a supporting position as in an existing leaf spring within this narrow gap to design a spring having spring rigidity and hydraulic characteristic (mainly pressure loss), which an existing supporting structure has.